AccountList.cls

public with sharing class AccountList {
    public static List<Account> getAccounts() {
        List<Account> accs = [SELECT Id, Name FROM Account];
        return accs;
    }
}

AccountList_Test.cls

@isTest         
public with sharing class AccountList_Test {
    @isTest static void testGetAccounts() {
        Test.startTest();
        List<Account> accs = AccountList.getAccounts();
        System.assertNotEquals(accs, null, 'getAccounts returns null');
        Test.stopTest();
    }
}